The present invention relates generally to the field of semiconductor devices, and more particularly to formation of electrical contacts for semiconductor devices.
Copper has a tendency to diffuse into adjacent material layers, such as the insulating layers that copper interconnects are formed in. Thus, diffusion barriers are used to prevent the diffusion of copper. Typical diffusion barrier materials are metals or metal nitrides, as examples. Because these materials have a higher resistance than copper, the diffusion barriers are typically made very thin to avoid excessively increasing the resistance of conductive features. However, such thin diffusion barriers have a tendency to form weak spots and holes, which can permit copper to diffuse into adjacent material layers.
One possible consequence of diffusion of metal from one layer of semiconductor material to another, in a semiconductor device, is the alteration of the effective work function of one or both layers. Most semiconductor materials have a carefully selected and controlled elemental composition and stoichiometry. If either the elemental composition or stoichiometry of such semiconductor materials are altered, then the semiconductor device may not function as intended, if at all. As such, the use of diffusion barriers is prevalent in the fabrication of many semiconductor devices. However, diffusion barriers occupy space between adjacent layers of semiconductor material, which increases the size of semiconductor devices. Their formation requires additional fabrication steps. In addition, inclusion of diffusion barriers often alters the effective work function of semiconductor devices. For at least that reason, diffusion barriers are typically made as thin as possible.